The liner of a refrigerator is in contact on one side with the food inside the refrigerator and on the other side with the insulation foam. Said insulation foam, typically a polyurethane foam, is made with a blowing agent which can cause environmental stress cracking (ESCR) of the liner. ESCR is the formation of cracks in a material caused by relatively low tensile stress and environmental conditions. The blowing agent can cause liner blistering, catastrophic cracks, tiny cracks (crazing) and loss of impact properties (embrittlement), as well as stress whitening and/or dissolution. The following prior arts relate to this subject.
U.S. Pat. No. 5,221,136 describes a refrigerator comprising an ABS or HiPS liner but said liner comprises a barrier layer on the side facing the polyurethane foam. The barrier layer comprises a polymer or copolymer of ethylene or propylene containing 0 to 40% by weight of a block copolymer rubber.
U.S. Pat. No. 5,834,126 is similar to the one above but the composition of the barrier layer resistant to the action of polyurethane foam blowing agents has an effective amount of a polyethylene modified with a compound such as maleic anhydride, maleic acid, maleic anhydride derivatives, maleic acid derivatives, or mixtures thereof and an effective amount of a rubber. The barrier layer composition may contain polyethylene, polypropylene, polybutylene, or copolymers thereof.
Other prior arts are dealing with rubber modified monovinylaromatic polymers which are deemed to be of interest in a lot of applications including the refrigerators but are not specific to the refrigerators.
U.S. Pat. No. 6,706,814 describes a rubber modified monovinylidene aromatic polymer comprising:
a) a monovinylidene aromatic polymer matrix; and
b) rubber particles dispersed therein, characterized in that the rubber particles are produced from a diene rubber having substantially linear structure containing less than one long chain branch per 10,000 carbon atoms in the polymer backbone with a solution viscosity of 5 cPoise to 1,000 cPoise and a Mooney Viscosity of 5 to 120.
The rubber particles are dispersed in the form of small and large particles, wherein the volume average particle diameter of the small particles is from about 0.1 to about 2 micrometers and volume average particle diameter of the large particles is from about 2 to about 6 micrometers. A lot of applications are described including refrigerators and freezers.
U.S. Pat. No. 6,545,090 describes a rubber modified monovinylidene aromatic polymer comprising:
a) a monovinylidene aromatic polymer matrix,
b) rubber particles dispersed therein, characterized in that the rubber particles are produced from a diene rubber having I) a high molecular weight component and II) a low molecular weight component; the high molecular weight component having a weight average molecular weight at least two and one half times greater than the weight average molecular weight of the low molecular weight component and the low molecular weight component constitutes from about 20 to about 80 weight percent of the total rubber content, wherein both components I and II have a 1,4 cis content of greater than 70 percent and III) the rubber is grafted using a graft promoting chemical initiator, with monovinylidene aromatic polymer to the extent that the amount of grafted rubber is at least 30 percent of the total rubber at phase inversion.
The rubber particles are dispersed in the form of small and large particles, wherein the volume average particle diameter of the small particles is from about 0.1 to about 2 micrometers and the volume average particle diameter of the large particles is from about 2 to about 6 micrometers and the small rubber particles are from 20 to 80 weight percent of the total rubber. A lot of applications are described including refrigerators and freezers.
U.S. Pat. No. 6,441,090 describes a rubber modified monovinylidene aromatic polymer having a bimodal particle size distribution comprising:
a) rubber particles of a star or branched low viscosity rubber having a volume average particle size of from 0.1 to 2 μm, and a cellular or core shell morphology or mixture thereof, and
b) rubber particles of a star or branched low viscosity rubber, linear diene rubber or block copolymer rubber having a volume average particle size of from 0.5 to 10μ,
characterized in that the rubber particles of b) are more dense than the rubber particles of a), having a smaller occluded monovinylidene aromatic polymer content than the particles of a), wherein the particles of a) are from 50 to 99 weight percent of the total diene rubber content. These rubber-reinforced bimodal compositions are described as useful in a wide variety of applications such as consumer electronics, small household appliances, toys and furniture. These polymers are also deemed to be useful in extrusion applications such as in the preparation of a gloss layer using coextrusion techniques for refrigerator liners.
U.S. Pat. No. 7,115,684 describes a mass polymerized rubber-modified polymeric composition comprising: a continuous matrix phase comprising a polymer of a monovinylidene aromatic monomer, and optionally, an ethylenically unsaturated nitrile monomer, and discrete rubber particles dispersed in said matrix, said rubber particles produced from a rubber component comprising from 5 to 100 weight percent of a functionalized diene rubber having at least one functional group per rubber molecule capable of enabling controlled radical polymerization;
wherein the composition is further characterized by:
    a) a volume average rubber particle size of from about 0.15 to 0.35 micron,    b) a total rubber phase volume between 12 and 45 percent, based on the total volume of the combination of the matrix phase and the rubber particles;    c) a partial rubber phase volume between 2 and 20 percent characterized by rubber particles having a volume average particle size of greater than 0.40 microns; and    d) a crosslinked rubber fraction of at least 85 percent by weight, based on the total weight of the rubber particles.These rubber modified polymers can be used in a variety of applications including injection molding and thermoforming of refrigerator liners, household appliances, toys, automotive applications and furniture.
Another technical problem is the environmental stress cracking resistance of the plastic material induced by localized sharp temperature variations and/or the presence of fats and oils in food or any aggressive chemical agent that may get in contact with the plastic material.
It is proposed to provide a plastic sheet structure to be thermoformed into a refrigeration liner that is resistant to chemical attack.
It is an object of the invention to provide a refrigeration appliance liner to be fabricated from a thermoformable, plastic sheet material exhibiting resistance to chemical attack e.g. blistering, cracking, crazing, as mentioned above, by polyurethane foaming agents.
It is an object of the invention to provide a refrigeration appliance liner to be fabricated from a thermoformable, plastic sheet material which retains a high level of toughness (impact properties) and strength (tensile properties), even at low temperatures (−20° C. or less).
It is another object of the invention to provide a liner made from a plastic sheet material that maintains processability similar to HIPS or ABS, including favorable extrusion conditions and similar thermoforming behavior.
It has now been discovered a refrigerator interior liner made of a rubber modified monovinylaromatic polymer composition which at least fits one of the above criteria. In brief the main features of this polymer composition are (i) a large size of moderately crosslinked rubber particles combined with a monomodal particle size distribution and (ii) a rubber phase volume fraction (RPVF) of at least 39%. The broadness of the rubber particle size distribution estimated by the RPS volume-to-RPS surface ratio, is preferably below 2.0, more preferably below 1.5 and most preferably equal or below 1.4. RPS volume means the volume median particle size, RPS surface means the surface median particle size of the rubber.
Secondarily, the average molecular-weight in weight of the PS phase should be sufficiently high, whereas the global concentration of low-molecular weight plasticizers such as white mineral oil and PS oligomers should be kept low-to-moderate.
The present invention also relates to a process to make said polymer. The process for making HIPS is well known to those skilled in the art and consists of polymerizing styrene monomer in the presence of dissolved rubber. Polymerization of styrene, and optionally a comonomer, is initiated by heating and/or by an initiator, by way of example a radical initiator. The rubber is “dissolved” in the styrene monomer (actually the rubber is infinitely swollen with the monomer). The usual rubber types utilized in the manufacture of HIPS include polybutadiene (PB), styrene-butadiene rubber (SBR), and styrene-butadiene-styrene rubber (SBS). Polystyrene is initially formed from the styrene monomer within the homogeneous rubber solution in styrene. At the beginning of the polymerization the reacting solution is at a point prior to the rubber/styrene inversion point, i.e. the point at which the solution being reacted goes from polystyrene particles in a rubber/styrene monomer matrix to rubber particles in a polystyrene matrix. When the degree of polymerization is about equal to the weight % of rubber in the system, it inverts e.g. the styrene/styrene polymer phase becomes continuous and the rubber phase becomes discontinuous. Styrene is polymerized around and within the rubber particles which leads to polystyrene inclusions in the rubber particles. A portion of the styrene is polymerized by grafting on the rubber, another portion is homopolymerized, said portion is referred to as a “non-grafting” polymerization. In HIPS a part of the styrene may be replaced by unsaturated monomers copolymerizable with styrene such as other monovinylaromatic monomers, alkyl esters of acrylic or methacrylic acid and acrylonitrile. The same mechanism of “grafting” and “non-grafting” occurs with the styrene comonomer, which means one portion of the styrene and of the comonomer are polymerized by grafting on the rubber, another portion of the styrene and of the comonomer are copolymerized. The properties of HIPS are related to the amount of rubber, the type of rubber, the rubber particles size distribution and volume fraction as well as the polystyrene included in the rubber particles. The proportion of styrene, and the optional comonomer, which is grafted (polymerized by the “grafting” way) is linked to the functionalization of the rubber. Rubber-modified vinylaromatic polymer compositions are well-known in the prior art. Composition fine-tuning so as to reach well balanced physical properties remains however a matter of know-how. Apart from the control of the phase rheological behaviours, the control of rubber phase grafting that occurs typically in situ during the conventional radical HIPS process is a challenge hard to overcome. Rubber phase grafting is indeed usually adjusted through the addition of organic peroxides, generating preferably H-abstracting radicals by thermal decomposition, and well-chosen according to their half-life decomposition temperature and the reactor temperature settings. However, the in situ rubber grafting during the HIPS process remains intrinsically a random reaction.
The main features of the process of the present invention are the use of a grafting initiator, a viscous rubber essentially of linear structure and the use of a chain transfer agent before the phase inversion. Additionally, the process as described in the present invention is performed in one or several polymerization reactors, under batch-wise or continuous polymerization conditions, with preferably limited back-mixing conditions within the inversion reactor—i.e. the reactor within phase inversion takes place—and within the just-former and just-subsequent reactors, and with also limited spillback flows between these aforementioned reactors.
EP 1201693 A2 has already described similar compositions for food containers and trays for a refrigerator but not for the liner. Moreover this prior art is silent on the ESCR of the liner due to the blowing agent of the insulation.
WO 94 12551 A1 describes impact styrenic polymers having good ESCR properties and improved physical properties to be employed in the production of thinner sheet stock for use in the manufacture of, for example, refrigerator liners, thereby resulting in reduced liner manufacturing costs. The compositions have a volume average particle size of at least 4 microns and a melt strength of at least 4.5 grams. There are no specific requirements for the rubber and nothing is mentioned on the broadness of the rubber particle size distribution.